The art of continuous casting has developed over the past several years so that sheet materials of all widths and thicknesses as well as continuous bars can be made in a highly efficient manner by the process. As is well known in the art, a pair of moving casting members, such as casting rolls, receive a metal bead fed from a nozzle on one side of the nip and by quickly cooling the metal, solidification takes place and a solid sheet or bar emerges on the other side of the nip. After further cooling the product is immediately ready for cutting into lengths or rolling into a roll for storage. Continuous casting has substantially revolutionized the metal industry in terms of cutting the cost of producing metal and in making higher quality stock.
One successful process and apparatus for continuous casting of sheet material is shown in the Harvey et al U.S. Pat. No. 3,405,757, issued Oct. 15, 1968. This patent describes a highly efficient method of casting sheet in continuous form that is free of imperfections, such as defects caused by entrapped gases and cross-banding caused by pulsations in the molten metal entering the nip between the casting rolls. Thus, a sheet with perfectly smooth surfaces and exact cross sectional dimensions is produced by the Harvey et al system. It is from this system that the preferred embodiment of the present invention departs and upon which improvements are made and the disclosure of this of this patent is incorporated in this application by specific reference.
The improvements of the present invention fill a long felt need for economic production of patterned sheets of metal, especially of aluminum alloy, that was not satisfactorily met by prior technology. That is, heretofore, where a pattern of any design was to be placed on the surface of a metal sheet, it had to be performed in a costly separate process step of rolling or embossing. The undue work hardening of the metal resulting from the extra heating that was necessary also tended to weaken the sheet and often resulted in premature failure due to cracking especially when dynamic loads were present. In embossed sheet, the back is not smooth, thus hindering attachment to other surfaces and not affording the increased strength that solid projections afford. In the decorative field, there is a need for substantially rigid sheets with a patterned surface for self-supporting curtain walls, exterior textured walls and integral interior wall paneling. The same sheets have mechanically functional uses as tread plates, metal stairs and machine housing to name a few.
The advance in the art of the present invention over continuous sheet casting procedures, including the Harvey et al patent, resides in the discovery that a sheet may be produced with a pattern of projections or nipples (and adjacent recesses) on at least one surface of the sheet without the difficulties previously considered by the experts to exist. It has been found that a substantially rigid metallic sheet (approximately one-sixteenth to one inch thick or higher) may be cast with projections by use of casting members that have recesses formed in the surfaces thereof, which recesses make the distances between the rolls much greater than previously found to be operable. It has been surprisingly discovered that the sheet may be made a desired nominal thickness with a nozzle designed for this thickness and with the same nozzle the projections that are intermittently presented at the nip between the casting members can be successfully filled with the solidified metal. This has been done without creating the problems of crossbanding, cracking and imperfections and loss of the integrity of the plenum chamber that has previously been held by the experts to foreclose the manufacture of sheets of varying cross section by a continuous casting process. To put it another way, producing a sheet of cross section that varies on a continuous casting machine has previously been rejected out of hand by those skilled in the art since the nozzle is normally closely and precisely related to the specific distance between the rolls just upstream of the nip of the casting rolls. In previous experience, if this distance between the rolls is interrupted so that suddenly a greater distance is provided, the molten metal immediately spilled and the gap between the nozzle and the rolls where the increased distance is provided fills with metal and the continuous casting system had to be shut down.
It has been specifically also discovered, much to the surprise of the experts, that where a pattern of projections is desired the parameters of molten metal temperature and pressure, cooling and speed of the rolls can be controlled so that a meniscus is formed that, in turn, retains the integrity of the molten metal between the nozzle and the surfaces of the casting members or rolls. Also, a gap is preferred between the sides of the nozzle and the rolls since the highly sensitive surface of the rolls is not subject to wear and less power is required to drive the rolls due to the reduction in friction. Furthermore, if molten metal is fed into the plenum chamber at the nip of the rolls in order to fully fill the chamber rather than relying on meniscus control of the bead, then the projections would be formed by rapid movement of the metal into the recesses on the roll as soon as the recess is first uncovered, which causes a disruption of the smooth distribution, resulting in deleterious trapping of gases and even extension of the metal behind the face of the nozzle where harmful sticking is likely.
In the field of thermoplastic continuous sheet forming, there have been successful formations of webs on a patterned roll. This has been possible in such patents as the Fields et al U.S. Pat. No. 3,515,778, issued June 2, 1970 since the cohesiveness of the material is quite different from that of metal. In working with plastic, there is no problem of the material becoming molten or liquid so as to fill the recesses in an uncontrolled manner. The thermoplastic resin will not flow out of the recesses on the patterned roll and thus the problem is not encountered as in metal forming. The U.S. Pat. to Kindseth No. 3,085,292, issued Apr. 16, 1963 shows how thermoplastic resin is self-supporting and thus can be fed onto a patterned roll without concern for free flowing material breaking loose and thus interrupting the continuous forming process.